The current invention is related to improvements in drill bits. And more particularly, it is related to improvements in drills for accurately starting a hole to be followed by the finishing drill. Additionally, a drill geometry is defined for a drill capable of drilling through difficult or hardened materials.
Drilling is one of the most common operations to be performed in a machine ship. As drills commonly have a large length to diameter ratio, the common accepted practice is to either center drill, center punch or to utilize a spotting drill prior to drilling to prevent the subsequent drill from walking off its location. The use of center drills, however, is limited to applications where the tool is perpendicular to the surface, as they are not particularly effective on inclined, angular, fractured, or off center curved surfaces, especially in small sizes or as the work material increases in hardness. Additionally, if a hole is started in the wrong position, it is very difficult to get the drill to accurately start at the correct location.
And as well, materials are often encountered that are difficult to work with. This may lead to breakage of a drill, or more commonly, a tap in the workpiece. This can create a real problem to remove as the broken tool is itself made of a hardened material which defies conventional tools. In addition, the fractured surface of the tool is probably the most difficult surface in which to start a drill. Carbide tools have had limited success in drilling out these materials as the interrupted cut encountered wreaks havoc on brittle carbide, resulting in either fractured cutting edges or a broken tool. Additionally, restoring the cutting edges of these tools is not particularly easy and generally requires special equipment.
Industry has successfully solved the problem with the use of a non-traditional machining method known as electrical discharge machining. However, as the equipment is expensive, as is the hourly rate charge, most shops do not have the equipment and are faced with either scrapping the part or sending it out, incurring the added expense and loss of time. It is, therefore, desirable to be able to drill out these broken tools, especially as the workpiece may already be set up on the machine tool. And removal rates are much higher by drilling.
Then, too, there are non-metallic materials such as ceramics, granite, or glass that are difficult or “undrillable” with conventional drills, and these materials, being electrically non-conductive, are not candidates for EDM machining. The ability to modify such materials is highly desirable as the items would otherwise have to be produced from a green stage prior to heat treatment, which may often distort the part.